Here in the original manuscript, Oliver Cowdery wrote “they saw him a coming”. At some later time, Oliver crossed out the prepositional a, perhaps when he copied the text from 𝓞 into 𝓟 (the a is lacking in 𝓟). The ink flow for the crossout of the a here in 𝓞 is very heavy and distinctly different than any other correction on this page of 𝓞. The crossout, in other words, appears to be due to editing. The critical text will restore the a in this instance.
The original text had a good many instances of the archaic, now dialectal, use of the prepositional a. For each instance of a supported by the earliest textual sources, the critical text will restore the a. See, for instance, the discussion under 1 Nephi 8:28 and Alma 28:5. For a complete list of this usage in the original text, see under prepositional a in volume 3.
Summary: Restore in Alma 55:8 the prepositional a that was originally in 𝓞 but was later consciously removed by editing.